1. Field of the Invention
This invention relates to a batchwise reduced-pressure fryer machine of the sort employing a food container case for holding therein various food products to be fried, including vegetables, fruits, sea foods and live-stock products as well as moldings or shaped products of these food stuff in the form of slices or chips, sticks, cubes or other arbitrary shapes, the food container case being designed to be immersed in heated oil of a reduced-pressure frying vessel to let the food product in the case undergo a predetermined frying process, and more particularly to a reduced-pressure fryer machine employing an improved food container case which is arranged to prevent the individual pieces of the food product in the case from abruptly floating up and sticking to a lid of the case or from sticking to each other to form larger lumps.
2. Description of the Prior Art
The reduced-pressure fryer machine of this sort generally includes a frying vessel, which is connected to a vacuum pump, and a food container case in the form of a metal wire basket or a box of punched or perforated metal sheet. The food container case which is filled, for example, with a predetermined amount of sliced food material is introduced into a frying vessel and immersed in heated oil for a preset time period under reduced pressure condition to fry the food material in the case.
In such a reduced-pressure fryer machine, however, it is often experienced that, when heated under reduced pressure, the food material in the case releases a great deal of steam vigorously for a certain time period in an initial stage of a frying process, and the released steam climbs up within the case and gushes out through the perforations in the lid toward the oil surface. The released steam is then discharged from the frying vessel by the action of the vacuum pump. Partly because of the energy of the vigorously climbing steam and partly because of the buoyancy of the food product itself, the individual pieces of the sliced food material in the case are urged to float up altogether and as a result forcibly pushed against the lid of the case, giving rise to problems such as adhesion of the sliced food material on the lid or agglomeration of the chips which will adversely affect the quality of the ultimate products.
Therefore, there have been proposed various countermeasures in this regard, for example, a method of supporting the food container case on a wag frame which is vertically shaken in the oil during a frying operation, or a method of repeatedly moving the food container case into and out of the oil for a few minutes before immersing the case in a still state in the oil. The first-mentioned method has a problem in that it is extremely difficult to disperse the sliced food material without adhesion or agglomeration because the whole content of the case tends to float up under the influence of great buoyancy as soon as the case is immersed and shaken up and down in the oil. This tendency is observed more conspicuously especially with a larger content of the processing food material. The second one of the above-mentioned methods also has drawbacks in that the vertical movements of the container case into and out of the oil in the course of a frying operation normally result in an inferior heating efficiency due to intermittent heating and an elongated processing time, in addition to the difficulty of giving a finish of a soft-and-full touch to the products and degradations in yield due to breakage losses of the products taking place while the case is moved up and down over a large stroke.
The above-mentioned phenomenon of the food material being pressed against and sticking on the lid of the food container case in the course of a frying operation as well as the phenomenon of the food material sticking together and agglomerating into lumps is considered to be mainly attributable to the construction of the food container case itself. Namely, normally the steam which is released from the food material in the frying process tends to climb straight upward. Therefore, it is presumable that, in a conventional food container case which is formed with a multitude of pores or perforations, the released steam from the food material vigorously flows out of the case toward the oil surface through the lid on the top side of the case, causing the food material to float up all of a sudden under the influence of the energy of the climbing steam and the buoyancy of the food material itself, and pressing the food material strongly against the lower surface of the lid in multiple overlapped layers until they completely block the perforations in the lid. As soon as the lid perforations are blocked by the food material being fried, the climbing steam which is now unable to flow out smoothly through the lid tends to find flow passages through side walls of the case. In such situations, the steam once flows upward to hit against the food material and then turns into sideward directions, leaving the food material still stuck on the lid of the food container case.
Therefore, it is conceivable that the above-mentioned problems of the conventional food container cases could be eliminated by providing a case which is constructed to control the climbing steam flows in such a way as to let part of steam linger in an upper portion of the case and form there a buffering steam layer with a cushioning effect on the food material being hit against the lid of the case, while letting the remainder of the steam form irregular sinuous flows within the container case to encourage convectional flows of the heated oil. The above-mentioned steam layer, coupled with the stirring action on the food material by the convectional heated oil flows, is considered to contribute to the eliminate the above-mentioned problematic phenomena, i.e., the adhesion of food material on the case lid and the agglomeration of food material into lumps.